(a) Field of the Invention
The present invention relates to a transmitting and receiving method in an OFDMA (orthogonal frequency division multiple access) system. More specifically, the present invention relates to a method for allocating pilots and data for improving frequency reuse rates in an uplink of an OFDM (orthogonal frequency division multiplex) system.
(b) Description of the Related Art
The present invention relates to a method for allocating pilot subcarriers, and a method and device for transmitting and receiving pilot subcarriers in an OFDMA (orthogonal frequency division multiple access) system. More specifically, the present invention relates to a method for allocating pilot subcarriers of a base station for improving frequency reuse rates in an OFDM (orthogonal frequency division multiplex) system.
(b) Description of the Related Art
In order to realize a BMWS (broadband multimedia wireless service) system which enables reliability of high speed and large-capacity services, OFDM transmission methods for transmitting signals with high data rates in the millimeter wave bandwidths of from several to several tens of GHz have been used.
The OFDM method represents a frequency multiplex system for perform an IFFT (inverse fast Fourier transform) on the data to be transmitted, dividing available bandwidths into a plurality of subcarriers, transmitting them, allowing an OFDM receiver to perform a FFT (fast Fourier transform) on the transmitted subcarriers, and converting them into original data, and it also represents a multiplex communication system for providing a specific orthogonal condition between subcarrier frequencies, and separating respective subcarriers from the receiver irrespective of spectral superposition.
FIG. 1 shows a block diagram of a conventional OFDM system, and configuration and operation of a transmitter and a receiver of the OFDM system will be described with reference to FIG. 1.
An OFDM transmitter 10 comprises a serial/parallel converter 2, a modulator 4, an IFFT (inverse fast Fourier transform) unit 6, a parallel/serial converter 8, and a D/A (digital/analog) converter and filter 12.
The serial/parallel converter 2 converts high-speed transmit data received in series into low-speed parallel data.
The modulator 4 modulates the data parallel-converted by the serial/parallel converter 2 through a predetermined modulation method.
The IFFT unit 6 transforms the data modulated by the modulator 4 into signals on the time axis, and outputs results.
The parallel/serial converter 8 converts the parallel data output by the IFFT unit 6 into serial signals.
The D/A converter and filter 12 converts the serial signals output by the parallel/serial converter 8 into analog signals, filters the analog signals, and outputs filtered results to the receiver through an RF (radio frequency) terminal.
That is, the data symbols output by the serial/parallel converter 2 are modulated by corresponding carriers, OFDM symbols are configured through the IFFT unit 6, and are finally input to the RF terminal transmitted to channels.
Also, the OFDM symbols are transmitted per symbol unity, but they are influenced by previous symbols while being transmitted through a multipath channel. In order to prevent OFDM inter-symbol interference, a CP (cyclic prefix) is provided to the parallel/serial converter 8 so that the CP may be additionally inserted between the adjacent OFDM symbols by establishing a length of the CP to be greater than the maximum delay spreading of a channel.
Next, the OFDM receiver 20 comprises an A/D (analog/digital) converter and filter 29, a serial/parallel converter 28, an FFT (fast Fourier transform) unit 26, a channel estimator 23, a demodulator 24, and a parallel/serial converter 22.
The A/D converter and filter 29 receives the analog signals from the transmitter 10 through the RF terminal, filters the received signals, and converts them into digital signals.
The serial/parallel converter 28 eliminates the CP inserted into the digital data converted by the A/D converter and filter 29, and converts them into parallel signals.
The FFT unit 26 performs an FFT on the time-axis data of the parallel signals converted by the serial/parallel converter 28, and generates frequency-axis data signals.
The channel estimator 23 estimates channel estimates of the frequency-axis data signals transformed by the FFT unit 26 for the purpose of synchronization demodulation of data.
The demodulator 24 uses the channel estimates found by the channel estimator 23 and demodulates the data.
The parallel/serial converter 22 converts the parallel signals demodulated by the demodulator 24 into serial signals.
Since the above-configured OFDM system parallels a predetermined data sequence by the number of subcarriers used for modulation, and modulates the corresponding subcarriers by using the parallel data, the total data rates maintain the original high speed, and the symbol period of the subchannels including the respective subcarriers is increased by the number of subcarriers.
Therefore, the frequency-selective multipath fading channel is approximated as a frequency-nonselective channel with respect to each subchannel, and corresponding distortions can be easily compensated by using a simple receiver.
As described above, the OFDM method has an advantage of reducing complexity of the receiver in the broadband transmission with severe frequency selective fading, and in order to reduce the complexity, the OFDM method uses the CP and eliminates influences caused by delay spreading.
However, as shown in FIG. 2, when the adjacent cells use the same frequency, signals of the first terminal and signals of the second terminal coupled to the first base station are provided to the second base station. Hence, the third terminal coupled to the second base station is interfered by the first and second terminals.
FIG. 3 shows a conventional pilot pattern of an upper frame using a preamble.
The conventional HyperLAN or the IEEE 802.16a uses a preamble to estimate a channel of an uplink.
When mobility of a terminal is provided as shown in FIG. 3, performance for channel estimation is degraded because initial channel estimates are time-varying. Also, when the positions of the terminals of an adjacent base station are provided on the border of the cell, preambles of the adjacent cell are collided and the performance of channel estimation is degraded since the positions of the preambles of all the base stations are the same.